It has become increasingly attractive to use alcohol fuels like methanol and ethanol in compression ignition engines instead of diesel fuel because use of alcohol fuels results in lower exhaust emissions. Alcohol fuels, however, cannot ordinarily be used in typical compression ignition engines because alcohol, unlike diesel fuel, requires constant use of an ignition source for combustion during normal engine operation. In the usual compression ignition engine, air is drawn into the engine cylinder and then compressed by the piston as the piston moves towards its top dead-center position. The compression causes the air temperature to increase, and when diesel fuel is sprayed into the combustion chamber, the fuel ignites upon contact with the heated air. Alcohol fuels, however, will not ignite in an ordinary compression ignition engine because alcohols generally require higher air temperatures for ignition than can be achieved by compression in the engine. Consequently, if alcohol fuels are to be used in a compression ignition engine, an ignition source like a spark plug or a glow plug must be provided in the engine combustion chamber during normal engine operation. Unfortunately, spark plugs and glow plugs are not very durable when constantly used in engines burning alcohol fuel and must be frequently replaced.
One method of causing alcohol fuels to ignite in a compression ignition engine without use of spark plug or a glow plug during normal operation of the engine is to provide the combustion chamber with a catalyst. Use of a catalyst allows alcohols to ignite at significantly lower temperatures than they would without the catalyst. However, the catalyst must be maintained at a sufficiently high temperature to be effective in causing the ignition of the fuel. The catalyst can be heated to the required high temperature by the heat generated from fuel combustion.
Use of a catalyst in a combustion chamber is not new. For example, three patents issued to Pfefferle, U.S. Pat. Nos. 4,811,707, 4,819,595, and 4,773,368, describe compression ignition engines in which the internal walls of the combustion chamber, including the head of the piston, are partially coated with catalytic material that is applied over an underlying layer of insulating material. A problem with using the Pfefferle engines for burning alcohol fuel is that the catalyst is coated on the walls of the combustion chamber and a system for cooling the combustion chamber walls would also cool the catalyst. Such cooling may prevent the catalyst from reaching and maintaining a sufficiently high temperature to be effective in igniting the fuel.
A further problem posed by the Pfefferle engines is the difficulty and substantial expense that would be involved in retrofitting existing compression ignition engines with a coating of catalytic material. Moreover, if the catalyst should need to be replaced, substantial expense and difficulty would be involved in providing a new coating of catalytic material.
Consequently, a need has arisen for a catalytic device that can be easily installed in existing compression ignition engines and that can be maintained at sufficiently high temperatures so that the engine can be operated effectively on alcohol fuels.